Unraveling surface and bulk dynamics of iron(III) molybdate during oxidative dehydrogenation using operando and transient spectroscopies

Iron(III) molybdate (Fe₂(MoO₄)₃) is a commercial catalyst for the oxidative dehydrogenation (ODH) of methanol, but it has recently been shown to be relevant for other substrates as well. Despite its commercial use, a detailed mechanistic understanding of Fe₂(MoO₄)₃ catalysts at the surface and in the bulk has been lacking, largely hampered by the lack of suitable spectroscopic methods, directly applicable under reaction conditions. Using propane ODH as an example, we highlight the potential of operando Raman and impedance spectroscopy combined with transient IR spectroscopy, to identify surface active sites and monitor the hydrogen transfer and oxygen dynamics. By comparison with the behavior of reference compounds (MoO₃, MoOₓ/Fe₂O₃) a mechanistic model is proposed. The presence of iron greatly influences the reactivity behavior via oxygen diffusion but is moderated in its oxidative capacity by surface MoOₓ. Our approach directly elucidates fundamental properties of Fe₂(MoO₄)₃ of general importance to selective oxidation catalysis.